In principle, yes. Rockbox has an MPEG2 decoder on it for instance. However, given the limited CPU power, you'll probably be limited to a fairly small subset of mp4 files if you want realtime playback.

[Thread Merged]
Rexracer, please use the appropriate suggestion thread in the future.
Unfortunately I do have to tell you that pretty much all of our old developers have stopped frequenting the forums since the device never gained any real height. Myself I have also stopped development for the X-Fi2 and moved on to Android devices. I stick around the forums for moderation duties and tech-support where I can provide it.

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